Avalonia/src/Avalonia.Themes.Default/Separator.xaml

<Styles xmlns="https://github.com/avaloniaui">

  <Style Selector="Separator">
    <Setter Property="Focusable" Value="False"/>
    <Setter Property="Template">
      <ControlTemplate>
        <Border BorderBrush="{TemplateBinding BorderBrush}"
                BorderThickness="{TemplateBinding BorderThickness}"
                Background="{TemplateBinding Background}"/>
      </ControlTemplate>
    </Setter>
  </Style>

  <Style Selector="MenuItem > Separator">
    <Setter Property="Background" Value="{DynamicResource ThemeControlMidBrush}"/>
    <Setter Property="Margin" Value="29,1,0,1"/>
    <Setter Property="Height" Value="1"/>
  </Style>

  <Style Selector="ContextMenu > Separator">
    <Setter Property="Background" Value="{DynamicResource ThemeControlMidBrush}"/>
    <Setter Property="Margin" Value="29,1,0,1"/>
    <Setter Property="Height" Value="1"/>
  </Style>

</Styles>